The present invention relates to an ultrasonic low range speed meter, and more particularly relates to an improvement in non-contact mode measurement of the linear speed of a mobile object which moves at a speed as low as feed speeds on a machining apparatus.
A coherent optical system is conventionally used for non-contact mode measurement of a mobile object. This system is based on the principle of optical interference. Although this system assures measurement with high precision, its application is more or less limited depending on the optical nature of the reflective face of the mobile object whose linear speed should be detected. In addition, the system is rather complicate in construction and, as a consequence, very expensive. For these reasons, this system is adopted in applications in which measurement with high precision is strongly required.
An ultrasonic Doppler system is another conventional example of the non-contact mode. In the case of this system, however, if the sound speed is 342 m/sec, the linear speed of a mobile object is 1 mm/min and the frequency of the sound wave used for measurement is 100 kHz. The resultant Doppler frequency is equal to about 0.005 Hz. In other words, only one measured value is obtained within a period of 200 sec. This slow pace of measurement is quite unsuited for practical applications in which quick response in process control is wanted. In addition, this system does not exhibit a high degree of resolution in the low speed range measurement.